1. Field of the Invention
The present invention relates to a color image processing system comprising a color image input device such as a scanner, a color image output device such as a display or printer, and a color image processing device, and to the color image input device and color image output device employed in the system. More particularly, this invention is concerned with a color image processing system in which color image information (data) defined in a color space inherent to each device is converted into color image information (data) defined in a common color space so that the color image information can be transferred freely within the system.
2. Description of the Related Art
In a color image processing system, how to reproduce desired colors is very important. A conventional color image processing system is constructed as a closed system comprising, in addition to an image processing device such as a workstation, a dedicated color image input device and color image output device such as a dedicated color scanner, color display, and color printer. This is because a color space used to express color image data to be handled by the color image input device or color image output device is defined, and color image data produced by a certain color image input device cannot be output as it is from another color image output device. Besides, since each device exhibits an inherent characteristic concerning colors, if color image data expressed in the same color space is reproduced as it is, original colors cannot be reproduced. The color image processing system therefore has a conversion feature for converting an input or output of each color image input or output device into suitable color image data. In this case, color image input devices and color image output devices are mutually associated on a one-to-one basis. The conversion feature can be designed so that color image data can be converted into color image data matched with the characteristic of each output device. Thus, color image information produced by a certain system is supposed to be output from the system but not to be used by another system. On the contrary, color image information produced by another system is not supposed to be used by the certain system.
In recent years, an image processing system has been requested to be a multi-vendor system in which color image input and output devices of numerous vendors (or providers of jobs relevant to color image processing) for handling colors are interconnected. Even in the closed system, as long as conversion is carried out, color image information produced by another system can be reproduced. However, it is necessary to optimize color image information for an individual system. This work is time-consuming and needs expertise. The work of color matching is referred to as color management. A system for use in carrying out the work of color matching is referred to as a color management system.
In the case of a multi-vendor system, all vendors do not use color image input and output devices that exhibit specified characteristics. It is requested that all vendors including the vendors, who do not use color image input and output devices that exhibit specified characteristics, can utilize a color image as it is. In this case, it is unknown what characteristic is exhibited by a device used by each vendor for producing or reproducing a color image. An actual color image has therefore been provided together with color image data. Based on the actual color image, the color management work that is time-consuming and needs expertise has been carried out. In the light of the advancement of computing and communications, a multi-vendor system making it possible to utilize color image information produced by a vendor in a remote place or enabling a vendor in a remote place to print produced color image information is feasible. In the case of this system, since an actual color image cannot be provided together, it becomes much harder to reproduce desired colors.
A system in which an output or input of each vendor is converted into color image data independent of a device and transferred has been devised. Color image data independent of a device is represented by associated data of colorimetric values measured under a given environment such as an XYZ space or Lab space. A system in which a conversion feature converts data defined in the XYZ or Lab space into another data is used to make color image data expressed in any of a plurality of color spaces usable may presumably be realized. Each of devices constituting the system includes a conversion feature for converting color image data inherent to the device into color image data expressed in a given color space. In a conventional color management system, information concerning colors handled by a device is managed using a conversion table for simply linking device-inherent data with colorimetric values or using a transformation expression such as a determinant.
In the foregoing color management system, once a colorimeter to be employed is defined, color image data to be transferred between devices should be data expressed in a color space independent of a device and correspond to the same color between the devices. However, when the system is operated actually by a user, there arises a problem that desired colors are not reproduced. Possible causes will be described below.
First, it is impossible for one colorimeter to measure colors handled by all input and output devices. For example, a calorimeter for measuring colors of a print or photograph cannot match colors for a color TV camera. Moreover, when a conversion feature is created by comparing the results of measurement of colors of a color chart by a colorimeter with the output of a color TV camera having imaged the color chart, the results of measurement and output vary depending on the conditions for illuminating the color chart. Moreover, there is a problem that the results of perception of the color chart vary depending on the conditions for illumination and are different from user to user. Another problem is that a print or photograph of the color chart is discolored. Furthermore, in practice, it is very hard for one colorimeter to match colors for all devices. Especially in the system for transferring color image data over a communication line, it is unfeasible for one colorimeter to match colors for all devices. In practice, there is great difficulty in matching colors handled by all devices with the same colorimetric values. Even when color image data expressed in the same color space is employed, colors are different among devices.
Secondly, perception of colors is delicately different from user to user. This depends greatly on an environment such as lighting and the user's psychological factors.
Any conventional system can have a problem that color image data is not provided as desired data or desired colors are not reproduced. In the aforesaid system, a conversion feature in each device is created on the basis of colorimetric values and produced on the assumption that color image data is independent of a device. A user is not supposed to adjust colors. However, in reality, since desired color data is not produced or desired colors are not reproduced, a user of a device adjusts a conversion feature used to convert color image data inherent to a device into color image data expressed in a specified color space, and thus adjusts colors in line with the user's environment or liking. However, the conversion table or determinant that is the conversion feature is used to merely convert color image data inherent to a device into color image data expressed in a given color space. The conversion feature is not created in consideration of human senses. This poses problems that it is very hard to adjust the conversion feature and that expertise and time-consuming work are needed for the adjustment.